The present invention relates generally to devices used to measure the level of a liquid within a vessel or container and more specifically to such a device formed of a thick film (e.g., ceramic) substrate bearing (e.g., screen printed) heaters and thermistors.
There exists a wide variety of applications in which it is desirable to monitor to some degree the level of a liquid within a vessel or the like. Such applications may range from monitoring various fluid levels within a motor vehicle or internal combustion engine or fuel tank to monitoring oil levels within a pump or compressor or even water levels within a tank such as for a recreational vehicle or boat. In each of these applications it is desirable that the liquid level sensor be capable of providing a reliable accurate indication of the liquid level over an extended period of time without requiring periodic maintenance. In many applications the level sensor must be capable of enduring various levels of vibration, heat or other hostile environmental elements as well as space limitations. Additionally, in some applications utilizing sealed vessels such as hermetic compressors, it is desirable to minimize the number of penetrations such as electrical leads through the walls of the vessel to reduce the potential for leakage.
Various types of devices have been developed over the years for sensing such levels of liquids. Such devices range from the extremely simple float arrangement commonly employed in fuel tanks to more complex electrical capacitance type sensors as well as microprocessor based thermocouple sensors. While operable, these various types of sensors have had varying drawbacks depending upon the particular application such as excessive space requirements for accommodating moveable floats and associated linkage, relatively high costs to manufacture, complexity of circuitry required to generate a level indicating signal, susceptibility to errors from extended or extraneous electrical noise, lack of resistance to hostile environmental elements, etc.
In many applications, where the level of a liquid needs to be monitored (and possibly controlled), it is not necessary to provide a continuous (analog) liquid level signal, but limit sensing to one (or two) discrete levels over a small level range. In this case, a ceramic substrate sensor with discrete thermistor/heater pairs provides necessary information to control the level of the fluid. Protection of the substrate from liquid (and potential contaminants) is accomplished by coating the surface with an inert glass (“glaze”) layer and/or polymer layer (e.g., “Parylene”). Packaging of the coated substrate is accomplished by protective base, epoxy, and slosh shield. Cost can be significantly reduced over a prior art multiple thermocouple based sensor design, since discrete electronic components are avoided. Also, the number of pins, and therefore electrical connections, can be significantly reduced.
Additional advantages and features of the present invention will become apparent from the subsequent description and the appended claims taken in conjunction with the accompanying drawings.